The two conventional forms of hexagonal boron nitride are turbostratic boron nitride and ideal or graphitic boron nitride. The hexagonal form of boron nitride is used in the conversion to cubic boron nitride and as a filler material for many other applications particularly where high thermal conductivity and high electrical resistivity is required. Typically, turbostratic boron nitride is first purified into what is conventionally referred to as "high purity hexagonal boron nitride" by treatment at high temperature, typically between about 1800.degree. C. to 1900.degree. C., for removing volatile impurities and surface oxide contaminants. Such high temperature treatment causes the boron nitride to become highly agglomerated in consistency which must be broken down for suitable commercial application. Accordingly, current practice is to first mill the high purity boron nitride into a fine powder and then, for ease of handling, to cold press and granulate the boron nitride in one or more stages. The milling operation forms a fine powder of small particle size typically with 99.9% of all of the milled powder below -325 mesh. The average particle size of the milled powder lies between 5-11 microns. The density of the boron nitride pellets formed from the cold pressing operation is no greater than an average of about 1.80 g/cc or 80% of the theoretical density of hexagonal boron nitride independent of the number of repeated granulation and cold pressing stages.
In the conversion of high purity hexagonal boron nitride to cubic boron nitride the compacts or pellets of boron nitride formed by compaction are subjected to extremely high pressures and temperatures within the stable region of the cubic boron nitride phase diagram. The density of the boron nitride pellets is significant to the economics of the cubic boron nitride conversion process.
It has been discovered in accordance with the present invention that the density of cold pressed boron nitride powder may be substantially increased to a density of at least about 1.86 g/cc and approximating 1.9 g/cc i.e approximating 85% of theoretical by controlling the particle size distribution of the boron nitride particles prior to compaction so that the distribution of particle sizes is as wide as possible and preferably with the majority of the particles having a particle size above 50 microns. The preferred particle size range for the majority of the particles should be between 20-500 microns. It has been further discovered in accordance with the present invention that agglomerated particles of boron nitride formed from a wide boron nitride particle size distribution following cold press compaction and granulation will possess a density closer to the average density achieved with hot pressing. In addition its thermal conductivity for use as a filler is enhanced particularly for use as a filler material in polymer composites.